To all motions by human in an upright position, like standing, sitting, running or making upright motions on an exerciser, the gravity has negative effects on the human body. These negative effects from the gravity include lumbar vertebra and cervical vertebra diseases, knee joint injury, visceroptosis, cardiovascular and cerebrovascular diseases, etc. Corresponding to the upright motion, the movement with most of human body in a horizontal plane is called horizontal motion. For example, swimming is a typical horizontal motion widely known. As the negative effects of gravity is greatly reduced when human body is in a horizontal plane, the horizontal motion is considered as a more scientific and reasonable motion. Therefore, various horizontal-motion exercising devices are developed continuously.
Inventions, such as “Mid-Body Exercise Device” disclosed in patent documents with publication number of U.S. Pat. No. 5,224,909A, “Exercise Machine for Back Rehabilitation” disclosed in patent documents with publication number of US20090105050A1, and “Prone Exerciser” disclosed in patent documents with publication number of US20100048367A1, are capable of ensuring users to do motions that mimic crawling and swimming in a horizontal plane. These motions are linear reciprocating motion, but the real crawling or swimming motion is not linear (when crawling, human's hand and knee move in an elliptical path). What is more, impact which easily occurs at two ends of the linear reciprocating motion results in an unsmooth and uncoordinated motion which may cause injury to human body.